MKAbeFook for Facebook. Uses the WebKit browser with asynchronous requests, photo uploads, and much more. It can call any API method and returns the result in standard Cocoa types. It is distributed under a BSD style license.
With Instagram's new App update, the "find your friends" on Twitter is very obviously missing. This is due to restrictions from Twitter's side, and seems to be a snub in Instagram's direction as many apps like Foursquare still have access to this part of the API.